We are studying the development of the hermaphrodite vulva in order to understand how cell-cell communication controls cell fates. In vulval development, Pn.p cells express any one of three potential cell fates (termed 1 , 2 or 3 ) in response to extracellular signals originating from the anchor cell, other Pn.p cells and possibly the hypodermis. Most of the genes that specify vulval cell fates fall into two categories; ten genes have a vulvaless mutant phenotype in which all Pn.p cells express a 3 cell fate and ten genes have a multivulva mutant phenotype in which all Pn.p cells express a 1 or 2 cell fate. We have focused our studies on
lin-31, because this gene does not fit into either of these categories. Mutations in
lin-31 cause Pn. p cell fates to become deregulated, since any Pn.p cell can express any one of the three cell fates. In
lin-31 mutants, Pn.p cells near the anchor cell can express the ground-state (3 ) cell fate and Pn.p cells far from the anchor cell can express an induced (1 or 2 ) cell fate, indicating that
lin-31 mutations do not simply activate (e.g. multivulva) or repress (e.g. vulvaless) the anchor cell inductive pathway. We propose that
lin-31 is a gene required to specify all three cell fates, and that
lin-31 plays a role in the determination of all six Pn.p cells. In
lin-31 mutants, Pn.p cell fate may become deregulated because the developmental state of each of the Pn.p cells (
lin-31 off) is never encountered in wild-type development. Our genetic studies indicate that
lin-31 acts at a late step in the vulval determination pathway. First, laser ablation experiments have shown that the
lin-31 mutant phenotype is independent of the anchor cell. Second, the
lin-31 mutant phenotype is epistatic to the vulvaless mutant phenotypes of
lin-2,
lin-7,
lin-10,
let-23 and
let-60. These data suggest that the ultimate function of the anchor cell signalling pathway may be to regulate
lin-31 gene activity. We have cloned
lin-31. We found that five out of seven mutatorinduced alleles have a Tc1 insertion in the
lin-31 locus. Unfortunately, we have not been able to find any RNA transcripts that are clearly affected by the Tc1 insertions. We are now using transformation experiments to identify the region of DNA containing
lin-31-rescuing activity. We have shown that a yac clone that includes the Tc1 insertion sites, Y14H12, is able to rescue the
lin-31 mutant phenotype. We are using recombination in yeast to rapidly generate smaller derivatives of Y14H12.